Method of reducing corrosion of ferrous metal surfaces by ammonium nitrate solution



IL a

states The present invention relates to the corrosion inhibition of ferrous metals. More specifically, the present invention relates to a composition and method of reducing the corrosion of ferrous metal surfaces in contact with aqueous ammonium nitrate solutions.

There is a well recognized corrosion problem in industries concerned with the manufacture, storage, transportation and handling of aqueous ammonia ammonium nitrate solutions. convenient to transport and storethem in ferrous containers such as drums, tanks and pipelines. However, in view of the corrosive nature of ammonia ammonium nitrate solutions on ferrous metals many manufacturers. now use storage and transportation facilities constructed: of aluminum. Aluminum is used because its oxide film renders the metal inert to attack by the ammonium nitrate solutions. The remedy, however, is a costly one. Corrosion inhibitors of one type or another have been suggested and attempted with varying degrees of limited success.

One answer to this corrosion problem presented by ammonia ammonium nitrate solutions is the utilization of coating materials so as to exclude the salt solution. from the ferrous metal surface. This technique has not been employed in ammonia ammonium nitrate solution service to any great extent for if any pin holes remain in the coating after application, or are formed therein during use, the salt solutions attack the metal surface at these points. The large amount of corrosion product formed in these areas causes the rest of the coating to peel and eventually the entire ferrous metal surface is exposed to the corrosive solution.

' Copending application Serial No. 149,186 in the names of Franklin MfWatkins and David B. Sheldahl, filed November 1, 1961, concurrently herewith discloses a relatively thin coating composition consisting essentially of minute particles of carbon in a binder chemically inert to ammonium nitrate solutions, which composition when placed on ferrous surfaces remedies the aforementioned problems and provides corrosion reduction. The effectiveness of the compositions of that application is due to the action of the carbon in inducing the formation of a gamma-R film on ferrous metal surfaces exposed by pin holes that develop in the coating or are initially present after application of the coating. This oxide film passivates the area and prevent-s attack by the ammonia ammonium nitrate solutions. Formation of this passive film is due to the fact that electrical potential existing between the exposed ferrous metal and the carbon particles contacting ammonia ammonium nitrate solutions is sufi'icient to create the current density and voltage required for passivation to occur. In ammonia ammonium nitrate solutions a current density in the range of approximately 0.1 to 1.5 ma./cm. is required for passivation and the resulting (working) voltage will generally be at least about 0.7, usually about 0.7 to .75.

Although the aforementioned carbon coating composition is elfectfve in creating the required voltage for passiv'ation of ferrous metal surfaces there is a possibility that the carbon surface may become polarized while immersed in the ammonium nitrate solutions whereby the In the handling of such solutionsituis voltage diiferenee between the anodic pin hole and the cathodic carbon surface is reduced. If this occurs the voltage would be reduced below values required for passivation and corrosion would begin at the pin holes at rates equal to or greater than the corrosion rates for freelycorroding ferrous metal.

We have now found that the possibility of the volt age falling below the passivation requirement in the carbon-containing coating compositions can be avoided by incorporating in the composition a minor effective amount of manganese compounds or chromate salts or mixtures thereof, which materials are insoluble'in ammonium nitrate solutions. The selection of a desirable chromate is guided by the requirements that it be relatively insoluble in the ammonium nitrate solutions and that the cation be cathodic to ferrous metal. Suitable chromates for use in the present invention are for instance, heavy metal, i.e. atomic number of at least 25, chromates such as'Pb, Ba and Zn chromates. The invention also envisions the use of manganese compoundssuch asthePb', Ba and Zn permanganates or manganese dioxide. The amount of salt incorporated 'in the coating composition willvary depending on the particular arn-".

monium nitrate solution employed and thepercentageof carbon present in the composition but will ord'narily fall within the range of between about 5 to 20% by The carbon employed in the coating composition can be carbon in its amorphous or crystalline forms, for example, charcoal or graphite and is present in a pulverized or finely divided dispersible state. Generally the size of the carbon particles will be in a range of about 10 to 200 microns, preferably 20 to 50 microns in diameter. These carbon particles are in direct contact with the ferrous surfaces, preferably sufiicient of the carbon particles are in direct contact with such surfaces to give the desired galvanic protection. Accordingly, in the thin coating, say of about 0.5 to 20 mils, preferably about 1 to 10 mils, thickness, in electrical contact with the ferrous metal surfaces, there is generally provided, for instance, at least about 5% by weight of the coatingof the carbon particles, advantageously at least about 25 or 35% to say about by weight. Preferably the amount of carbon is about 25 to 50%. The amount of carbon particles necessary or desired is dependent upon factors such as their surface area, the extent of their electrical contact with the ferrous surfaces, the rate of contacting the ammonium nitrate solution and the presence of other ingredients, e.g. corrosion inhibitors, etc. in the coating. Thus only those particles in such electrical contact and in electrical contact with the ammonium nitrate solution are providing the desired galvanic protection and effectively preventing corrosion. A given particle may not directly contact both ferrous metal surface and the nitrate solution but the contact may be established through intermediate electrically conductive materials such as other carbon p arficles. Addition of the manganese or chromate compound of the present invention is of further advantage in that it reduces the carbon content requirement of the coating compositions and provides effective coating compositions whose carbon content is substantially less than is ordinarily necessary in the absence of these addItives.

The carbon particles, and manganese compounds or chromate salts are held in coating position by an organic or inorganic binder such as an organic resinous material, paint or similar coating composition. Also the carboncontaining coating can be overlain by another coating composition if desired, although this is not necessary, and it is when the carbon particles in electrical contact with the ferrous surfaces are also in contact with the ammonium nitrate solution that the corrosion inhibitionis being afforded by the galvanic couple. Contact with" the solution may be through carbon particles at the outer surface of the coating or at holes or cracks in the coating which are present due to imperfections in the coating, wearing away of the coating, etc. A carbon particle in direct contact with another carbon particle in the coating can be considered as a single particle.

. If the viscosity of the binder component will permit, the carbon, manganese compounds or chromate salts may be incorporated directly in the binder component by slow addition and stirring. Preferably a suitable solvent, for instance, a ketone such as methyl ethyl ketone or an aromatic solvent such as toluol, xylol etc. is added to the binder to reduce its viscosity and facilitate dispersion of the carbon particles and depolarizer. After the carbon and manganese compound or chromate salts are thoroughly mixed with the binder, the solvent is then removed as by evaporation to produce the coating composition. By coating composition, we mean a composition that is directly self-adhering when applied to a surface.

The binder component of the coating composition can be any of the materials commonly employed as binders in paints and other coating products which are chemically inert to ammonia ammonium nitrate solutions and may constitute the balance, of the composition, generally about 15 to 35 weight percent. Examples of'suitable binder material are: organic resins such as epoxy polymers, vinyl copolymcrs, styrenebutadiene copolymers, vinyl chloride resins, polyurethanes, oil-treated isocyanates, acrylic resins, phenolic resins etc. inorganic binders suchas the silicates may also be used.

If desired the coating composition may include minor effective amounts or arsenates o'rarsenites essentially insolublein the amomnium nitrate solution but essentially solublein nitric acid. Addition of these compounds has been found to improve the stability of the passive films formed. They are generally empolyed in amounts of about 5 to 20% by weight-of the total composition.

The coating compositions of the present invention may also include other ingredients which are not deleterious tothe passivating activity of the carbon particles, for instance, ingredients commonly found in paints and other coating products as, for example, metal oxide pigments, oils, plasticizers, resins etc. In actual practice it may be found advantageous to just incorporate the carbon particles in, for example, a paint composition commercially prepared whose formulation includes a suitable binder for the carbon particles, particularly those formulations notedfor their superior abrasion resistant qualities and their strong adhesion to ferrous metal surfaces, and inertncss to ammoniacal ammonium nitrate solutions.

The ammonium nitrate solutions may vary considerably in composition. Although our system protects vessels containing aqueous ammonium nitrate solutions greater need and utility reside in protecting vessels employed to handle aqueous ammoniacal ammonium nitrate solutions. Generally representative of such solutions encountered in industry and which give rise to the corrosion problem discussed heretofore are those having approximately about 1 to 80 or more percent ammonium nitrate, usually at least aboutv 40 percent, preferably about 60 to 70 percent, about 5 to 35 percent free ammonia, and the substantial balance being water, for instance, about 5 to 25' percent water. These percentages are by weight. Especially useful solutions are those containing a ratio of free ammonia to water of at least about 1.5 to 1 by weight.

Even greater assurance of full protection for the metal surface can be obtained by employing, in addition to the coating composition of the present invention, a ferrous metal-carbon galvanic couple, that is, employing one or more elemental carbon cathodes in the aqueous ammoniacal solutions and the ferrous metal as the anode and having a metallic conductive path between the carbon and metal surface. Thus a ferrous metal-carbon cathode galvanic couple is established which generates sufficient current in situ, that is, without the use of an external current source to accomplish passivation of any areas of the ferrous metal that for some reason are not passivated by the coating composition of the present invention. More important, the ferrous metal-carbon galvanic coupie supplies sui'licient current to assist the maintenance of passivation acquired from the coating composition of the present invention.

The current density created by a ferrous meta -carbon galvanic couple is dependent on the surface area ratio of the active ferrous metal to carbon in contact with the solution. The active ferrous metal is the non-passivated ferrous surface in contact with the solution and thus the rate of addition of the ammonium nitrate solution may affect the extent of corrosion protection obtaine Accordingly, the surface area ratio of ferrous metal to carbon and/or rate of addition should be selected to produce a current density sufiicient to passivate the ferrous surface. For example, the current density can be increased by increasing the area of the carbon or by slowing the rate of filling of the container. When the ratio of surface area of the ferrous metal to carbon is about 1:1 to 15:1 sufiicient current density is generated even to passivate ferrous metal containers to which the ammoniacal solutions are rapidly added or to maintain passivity once passivation of said ferrous metal surface is accomplished. Much greater ferrous metal to carbon surface ratios say even up to about 200:1 and even more can be used when the ammoniacal solution contacts the metal slowly. In any event, the differences in electrode potentials at the initial contact of the solution with the electrodes, i.e. the working voltage, however obtained, should be sufiicient to provide current densities necessary for passivation of the ferrous metal. Ordinarily a current densitywhich falls in the range of at least about 0.1 to 1.5 or more ma./cm. is found sufiicient for passivation of the ferrous metal and the voltage resulting may be about 0.7 to .75 volt.

Although the carbon cathode can be suspended into the aqueous ammonium nitrate solution it is preferred that it be directly connected to the lower portion of the container, e.g. the bottom particularly in the case of containers of large size, such as storage tanks so that the current density generated by the couple is sufliciently great to passivate the ferrous metal as the solution contacts it. This construction maintains the carbon electrode in contact with the nitrate solution in the vessel. It has been found that connecting the carbon cathode, preferably a plurality of carbon cathodes to the bottom of a large container such as a tank car, required steel/carbon surface ratios are assured and passivation is accomplished.

The ammouiacal solutions may contain additives well known to the art as corrosion inhibitors in these solutions. Examples of these inhibitors are trivalent arsenic compounds, for example, arsenic trioxide; an arsenite such as sodium, potassium or ammonium arsenites and sulfides of trivalent arsenic; compounds which contain divalent sulfur linked to an atom of carbon with the remaining valences of the carbon atom linking the carbon atom to nitrogen as, for instance, carbon disulfide, thiocyanates, thiocarboxylic acids, thioamides, etc. (see US. Patent No. 2,220,059 to Herman A. Beekhuis et al.); and organic compounds having an SH and an OH group, for instance, as disclosed in US. Patent No. 2,613,131 to Marion D. Barnes et al. In fact the presence of these additives, particularly the arsenitcs, when ferrous metal surface is being passivated by the method of the present invention may enhance the passivation even in amounts far smaller than taught as effective by the prior art. The presence of compounds which provide the ammoniacal solution with copper and carbonate ion, for instance, basic cupric carbonate, have also been found to enhance the passivation. Not only is the presence of these additives of advantage in enhancing passivation but once passivation has been accomplished they act to further insure protection.

A particular effective inhibitor additive is the combination of trivalent arsenic compound, a soluble copper compound and carbonate ions as disclosed in application Serial No. 5,637 in the names of Paul Shapiro, David B. Sheldahl and Lawrence V. Collings, filed February 1, 1960, herein incorporated by reference. The soluble copper compound can be, for instance, the inorganic compounds such as cupric carbonates, hydroxides, sulfates, nitrates, etc. Of the many carbonate ion-producing compounds, the more particularly suitable are the inorganic compounds, for instance, alkali metal and ammonium carbonates. Preferably, the copper and carbonate components are provided by a single compound such as basic copper carbonate.

The following examples are included to further illustrate the present invention:

EXAMPLE I Steel coupons 1" x 5" x 3 were cleaned 'by sand blasting and coating by dipping into the compositions indicated in Table I below. Coatings were made by suspending graphite (325 mesh) with and without a chromate salt in a binder also identified in Table I below.

After proper curing time (about a week) the coatings were gouged with a sharp file, inducing two scratches in the criss-cross pattern extending the length of the coupon. The coupons were then placed in eight ounce French square bottles partially filled with about 100 mm. of aqueous ammoniacal ammonium nitrate solution so as to allow about half the coupon to extend above the liquid line. The steel coupons were connected to a Sheppard potentiometer for measuring potentials. After about a day or two, appearance and single electrode potentials indicated that the coupon was passive, some of the coupons were removed and while the untouched side was gouged in the manner described above. The aqueous ammoniacal ammonium nitrate solution contained 66.8% NH NO 16.6% NH and 16.6% H O. The results are shown in Table I below.

Table l as to allow about one-half the coupon to extend above the liquid line. The steel coupon was connected to a Sheppard potentiometer for measuring potentials. Within a short time, appearance and single elect-rode potentials indicated that the coupon was passive. The passivated coupon was subjected to a rugged electrical test of stability known to reduce most passive films instantly or in one or two minutes. In this test the passive film is physically contacted with a 2" No. 12 copper wire while immersed in the ammonia-ammonium nitrate solution. Contact With the copper Wire took 20 minutes to activate the passivated coupon, thus demonstrating the high stability of the passive film formed by the composition. After activation, when the copper wire was removed, single electrode potentials indicated that the steel coupon had become passive again. The copper wire was replaced and an additional 12 minutes was required to reactivate it. Upon removal of the copper wire, the steel coupon once again became passive.

This example demonstrates the highly stable passive films produced by the coating compositions of the present invention and in addition, the ability of the M-nO -containing composition toward recovering passivity.

It is claimed:

1. A method for reducing corrosion of ferrous metal surfaces by aqueous ammonium nitrate solutions which comprises coating said ferrous metal surface with a coating composition consisting essentially of at least about 5% by weight of minute carbon particles, about 5 to 20% by weight of a compound selected from the group consisting of MnO and a chromate salt insoluble in said ammonium nitrate solution and essentially the balance a binder chemically inert to said aqueous ammonium nitrate solution, and contacting said coating with an aqueous ammonium nitrate solution.

2. The method of claim 1 wherein the aqueous solution is an ammoniacal ammonium nitrate solution.

3. The method of claim 2 wherein the compound selected is zinc chromate.

4. The method of claim 2 wherein the compound selected is M1102.

5. A ferrous metal container containing an aqueous Percent pigment Colaiting Percent Binder Observations Carbon Other B 50is0cyanate Scratched prior to immersion. After about a day; corrosiion observed, coating began to pee C 62 38isocyanate D0.

D 42. 5 15.0 (ZnCr04) 42.51socyanate Scratched prior to immersion, no corrosion.

G 74 13BaOrO4 13-lacquer Scratched prior to immersion, no corrosion. Removed from solution, scratched While wet, returned to solution, no corrosion for over 3 months although lacquer had been softened and coating almost completely removed at this time.

The data of Table I demonstrate the advantageous effect of adding minor amounts of chromate salt to carbon-containing coating compositions.

The data also show that although lacquer has its shortcomings, it nonetheless held the carbon in place long enough to provide current to convert iron surface to iron oxide and thereby produce passivation.

EXAMPLE II A steel coupon 1" x 5" x ,5 was cleaned by sand blasting and coated while dipping into a composition consisting of 9% M110 71% carbon and 20% vinyl chloride resin binder. The coating was made by suspending the graphite (30 to 50 microns) and Mn0 in the vinyl chloride resin binder. After a proper curing time (about a week), the coated coupon was placed in a bottle partially filled with 100 mls. of the aqueous ammoniacal ammonium nitrate solution of Example I so References Cited in the file of this patent UNITED STATES PATENTS 2,366,486 Bruni et a1. Jan. 2, 1954 2,874,105 Young Feb. 17, 1959 2,902,390 Bell Sept. 1, 1959 3,011,862 Watkins Dec. 5, 1961 

5. A FERROUS METAL CONTAINER CONTAINING AN AQUEOUS AMMONIA-AMMONIUM NITRATE SOLUTION, SAID CONTAINER BEING COATED WITH A COMPOSITION CONSISTING ESSENTIALLY OF AT LEAST ABOUT 5% BY WEIGHT OF MINUTE CARBON PARTICLES, ABOUT 5 TO 20% BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF MNO2 AND A CHROMATE SALT RELATIVELY INSOLUBLE IN SAID AMMONIUM NITRATE SOLUTION AND THE ESSENTIAL BALANCE A BINDER CHEMICALLY INERT TO SAID AMMONIUM NITRATE SOLUTION. 